Link: http://www.biz.uiowa.edu/iem/
It's kind of hard to explain. You trade in the futures market just like you would a commodity such as sow bellies or oranges. The difference is the market you're trading in is politics and the candidates are the contracts, set to expire like any contract would on a set date. Global events help determine the contract cost so theoretically it's an absolutely free market.
I'm guessing I'm the only one that finds this interesting.